In a flat display, a backlight source is often used in combination with a spatial light modulator and a color filter to present full-color images. In an image sensor of a digital camera, a color filter is also used in combination with color difference calculation to reproduce the color of an original object. In larger systems such as a color video camera or a back projection TV, a three-plate or two-plate prism set or a color filter is used in combination with a collimated light source to present full-color images. When the color filter is used in such systems, because each shading pixel can only present a single primary color of the RGB three primary colors, about two-thirds of energy of the incident white light is absorbed, thus decreasing the efficiency of using the light and shortening the lifespan of the battery. In addition, fabrication of the color filter can be rather complex and more than one semiconductor photolithography processes are needed for each primary color, which results in a high cost.
Please refer to FIG. 1 to FIG. 3, which show a common light separation architecture used in conventional color camcorders. There are three types of light separation architectures, which are a three-plate prism-type optical system composed of a zoom lens 1, an infrared filter 2, a three-plate prism 3, a red light charge-coupled device (CCD) 4, a green light CCD 5, and a blue light CCD 6, as shown in FIG. 1; a two-plate dichroic prism-type optical system composed of a zoom lens 1, an infrared filter 2, a two-plate prism 7, a red-blue filter 8, a red-blue light CCD 9, a green light CCD 5, as shown in FIG. 2; and an optical system with single-plate color filter composed of a zoom lens 1, an infrared filter 2, a red-green-blue filter 10 and a red-green-blue light CCD 11, as shown in FIG. 3 Among which, both the optical systems shown in FIG. 1 and FIG. 2, that are designed to achieve light separation by the use of their prisms and optical interference films, are disadvantageous in their bulky sizes and complex structures with plenty of optical elements required. However, the optical structure shown in FIG. 3, which directly uses a color filter for light separation, can be suffered by its low optical efficiency.
Therefore, researchers all over the world are working tirelessly to come up with all kinds of new techniques for overcoming the aforesaid shortages. One such study is disclosed in a paper published in Journal of SID 16/8, 2008, by Philips Co., and also in a paper published in EURODISPLAY 2002, pages 339˜342, by IBM, both of which use a sub-wavelength structure for splitting an incident beam into multiple beams of various colors and then enable the resulting beams to be focused on their corresponding sub-pixels by the use of a micro-lens array, so that cooperatively are capable of working as a substitute for the conventional dye photoresist. However, they both suffer the following shortcomings:
(1) it is not a easy task for producing a large-area sub-wavelength structure whose pitch is about 320 nm;
(2) the resulting light emitting thereby has poor uniformity; and
(3) high production cost.
Moreover, in U.S. Pat. No. 5,615,024A, entitled “Color Display Device with Chirped Diffraction Gratings”, a blazed diffraction grating capable of acting in replacement of color filters for separating an incident beam into beams of primary colors is disclosed, in which the resulting beams are primarily first order diffraction beams. Accordingly, when the aforesaid structure is applied in display panels, the beam of one primary color should be directed to correspond to one pixel. However, by the usage of the first order diffraction beams, a large included angle will be formed between its incident beam and emitting beam so that the incident beam must be directed to enter the blazed grating by a larger angle so as to enable the resulting emitting beam to enter its liquid crystal layer following the normal of the same. On the other hand, if the incident beam enter the blazed grating perpendicularly, it will result the emitting beam to enter the liquid crystal layer in a large angle which will require to have additional refraction elements for correcting the deviation, otherwise, it can not be applied in thin display panels.
In U.S. Pat. No. 4,807,978, entitled “Color Display Device and Method Using Holographic Lenses”, a holographic lens set capable of acting in replacement of color filters for separating an incident beam into beams of primary colors is disclosed, in which the resulting beams are primarily first order diffraction beams. Accordingly, when the aforesaid structure is applied in display panels, the beam of one primary color should be designed to correspond to one pixel. As the color separation in the aforesaid U.S. patent requires the holographic lens set to be composed of three layers of holographic lenses, not only it is extremely difficult to fabricate, but also it is difficult to align the lens arrays precisely with respect to each other. In addition, as there is severe cross talk between the resulting beams of three primary colors, the use of such holographic lenses in color display device will suffer high noise.
In U.S. Pat. No. 5,764,389, entitled “Holographic Color Filters for Display Applications, and Operating Method”, a holographic set capable of acting in replacement of color filters for separating an incident beam into beams of primary colors is disclosed, in which first an incident beam is separated into beams of different spectral regions corresponding to the three primary colors by the use of a holographic color filter, and then another holographic color filter is used for deflecting the optical paths of the resulting beams in a manner that the beam of one primary color is directed to correspond to one pixel. Similarly, since there are multiple layers of holographic color filters used for achieving the color separation, not only the optical efficiency is poor, but also it is difficult to align the holographic color filters precisely with respect to each other.
In the image sensor disclosed in TW Pat. No. M249217, a set of lenses is used in cooperation with a prism set, as a substitute to color filters, for separating an incident beam into beams of primary colors while deflecting the optical paths of the resulting beams in a manner that the beam of one primary color is directed to correspond to one pixel. As the lens set is disposed on the prism set and the shape of the prism is comparatively unsymmetrical with respect to the optical field of the image sensor, it is practically infeasible despite of its good optical efficiency.
Therefore, it is in need of a color separation system capable of acting in replacement of the conventional color filters for its simplicity and high optical efficiency. In addition, the color separation system should be able to separating an incident beam into a red, a green and a blue light beam that are directed to enter a liquid crystal layer of a display panel in a vertical manner with satisfactory optical efficiency.